As an architecture for a receiver of a wireless communications system, a heterodyne system is widely known. The system is to convert an RF reception signal into a low-frequency signal (IF signal) using a frequency converter (mixer). An example of the arrangement and operation of a heterodyne receiver will be explained with reference to FIG. 12. The RF reception signal is amplified by an LNA circuit 8, and an image frequency component of the RF reception signal is removed by an image removing filter 9. Then, by a frequency converter 10, the signal is multiplied by an oscillation frequency component of a local oscillator 15 and converted into an IF signal. After an unwanted frequency component of the IF signal is removed by a bandpass filter 11, the signal is amplified to an appropriate level by an amplifier 12, demodulated by a demodulator 13, and output after an unwanted frequency component of the signal is removed by a lowpass filter 14.
A significant problem in the heterodyne system is an image signal. In the heterodyne system, high frequency and low frequency bands that are located symmetrically to each other on the axis of frequency with respect to the local frequency are converted to the same frequency band. Therefore it is necessary to remove an image signal that is located opposite to a desired reception signal on the axis of frequency with respect to the local frequency. FIG. 13 shows a spectrum of these signals. The most common way to remove the image signal is to provide a filter for image removing at a preceding stage of the frequency converter, as explained earlier. However, it is difficult to integrate such an image removing filter into a semiconductor IC. Consequently, the image removing filter prevents the reduction of the size and cost of a high-frequency receiver.
In view of this, an invention of an image removing frequency converter that does not require the image removing filter is disclosed in Japanese Unexamined Patent Publication No. 6-343088 (Tokukaihei 6-343088, published on Dec. 13, 1994) and Japanese Unexamined Patent Publication No. 9-64649 (Tokukaihei 9-64649, published on Mar. 7, 1997), for example. Basically, the image removing frequency converter applies respectively different phase shift operations to the desired frequency signal and the image signal, and cancels the image signal by adding to the image signal, a signal of reversed polarity with respect to the image signal. The phase shift operations are performed using a 90-degree phase shifter. If built in a semiconductor IC, however, the 90-degree phase shifter generates errors in output level and phase, due to process variation, a parasitic component, and the like, and these errors prevent sufficient canceling of the image signal. As a result, the yield of ICs is lowered.
Next, an example of a high-frequency transmitter will be explained with reference to FIG. 14. After an unwanted frequency component of a transmission baseband signal is removed by a lowpass filter 16, the transmission baseband signal is modulated by a modulator 17, and amplified to an appropriate level by an amplifier 18. Then, after an unwanted frequency component of the signal is removed by a bandpass filter 19, the signal is supplied to a frequency converter 20. The frequency converter 20 multiplies the input signal by a frequency component of a local oscillator 23, so as to convert the input signal into a high-frequency signal. After an unwanted frequency component such as an unwanted side band component of the converted high-frequency signal is removed by a bandpass filter 21, the high-frequency signal is amplified by a power amplifier 22 and output.
In such a high-frequency transmitter, the frequency converter 20 outputs in addition to the desired output signal, an unwanted side band signal at a position symmetrical to the desired output signal on the axis of frequency with respect to the local frequency. FIG. 15 shows a spectrum of these signals. In order to remove the unwanted side band signal component, the bandpass filter or image removing frequency converter is required. However, the bandpass filter and image removing frequency converter prevent reduction of the size and cost of the high-frequency transmitter, or lower the yield of ICs, as explained earlier.
As described above, a transmitter and receiver employing a frequency converter requires an image removing filter or an image removing frequency converter in order to remove an unwanted frequency component. However, the image removing filter is difficult to be integrated into an IC, thus becoming a problem in reducing the size and cost of the transmitter and receiver. Further, the image removing frequency converter requires a 90-degree phase shifter. This complicates the circuit, and increases the power consumption of the transmitter and receiver. Moreover, the 90-degree phase shifter generates a level error and phase error due to process variation, thus causing such a problem that the yield of ICs is lowered.